Meiosis
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AP Biology › Meiosis
During meiosis in a diploid cell, tetrads form and then align at the metaphase plate. The student notes that each homolog in a tetrad is attached to spindle fibers from opposite poles, while sister chromatids of each homolog share attachment to the same pole. Which outcome is most likely when the cell transitions into anaphase I?
Chromosomes replicate again to ensure each pole receives a complete diploid set.
Centromeres divide so chromatids can assort independently into four nuclei immediately.
Homologous chromosomes separate and move to opposite poles, reducing chromosome number in each resulting cell.
Tetrads dissociate without movement, and cytokinesis randomly partitions chromosomes into cells.
Sister chromatids separate and move to opposite poles, producing diploid daughter cells.
Explanation
This question assesses the skill of analyzing meiosis by predicting anaphase I events from spindle attachments. In a diploid cell, tetrads align at metaphase I with homologs attached to opposite poles and sister chromatids to the same pole, leading to homolog separation in anaphase I, which reduces the chromosome number in each resulting cell, as in choice B. This reduction division halves ploidy while keeping sisters together. The attachment details ensure proper segregation of homologs. A tempting distractor is choice A, which describes sister chromatid separation producing diploid cells, based on the misconception that anaphase I mimics mitosis rather than separating homologs. For meiosis questions, examine spindle connections to homologs versus chromatids to distinguish between division types.
In a diploid cell with $2n=6$, homologous chromosome pairs align at the metaphase plate during meiosis I. In one observed cell, all three maternal homologs face the same pole while all three paternal homologs face the opposite pole. The cell completes meiosis I and then meiosis II normally, with sister chromatids separating in meiosis II. Which outcome is most likely for the chromosome sets in the four gametes produced from this single cell?
Two gametes receive three maternal chromosomes and two receive three paternal chromosomes, each haploid.
All four gametes receive three maternal chromosomes only, because meiosis I determines chromosome origin.
All four gametes are identical because sister chromatids separate only in meiosis I.
Two gametes are diploid with six chromosomes and two gametes are haploid with three chromosomes.
Each gamete receives a mixture of maternal and paternal chromosomes due to crossing over in metaphase I.
Explanation
This question assesses the skill of analyzing meiosis by examining the independent assortment of chromosomes during meiosis I and its impact on gamete composition. In this scenario, with 2n=6, all maternal homologs align toward one pole and paternal toward the other in meiosis I, resulting in one daughter cell receiving all three maternal chromosomes and the other all three paternal chromosomes after separation. During meiosis II, sister chromatids separate normally, so each of these daughter cells divides into two gametes, producing two gametes with three maternal chromosomes and two with three paternal chromosomes. Each gamete is haploid with n=3 chromosomes, as the diploid number is halved through the meiotic process. A tempting distractor is choice C, which incorrectly suggests that crossing over in metaphase I mixes maternal and paternal chromosomes in each gamete, misunderstanding that crossing over occurs in prophase I and does not affect the overall origin of whole chromosomes in this alignment. A transferable strategy for meiosis questions is to track chromosome origins and counts through each division, distinguishing between homolog separation in meiosis I and chromatid separation in meiosis II.
In a species with $2n=4$, homologous chromosomes separate during anaphase I. During anaphase II, sister chromatids separate when centromeres divide. A student examines a cell in anaphase II and counts four chromatids moving toward each pole. Which outcome is most likely for the chromosome number in each gamete produced from this meiosis?
Each gamete will have $n=2$ chromosomes because chromatids become individual chromosomes after separation.
Each gamete will have $n=1$ chromosome because homologs separate in meiosis II.
Each gamete will have $n=4$ chromosomes because chromatids are counted as chromosomes only in meiosis I.
Each gamete will have $2n=8$ chromosomes because chromatids double the chromosome number in meiosis II.
Each gamete will have $2n=4$ chromosomes because sister chromatids separated in meiosis II.
Explanation
This question assesses the skill of analyzing meiosis by determining chromosome number in gametes after meiosis II. In a species with 2n=4, homologous chromosomes separate in meiosis I, leaving each meiosis II cell with n=2 chromosomes composed of two chromatids each, and the observation of four chromatids total in anaphase II indicates the separation into two per pole. As sister chromatids separate in anaphase II, they become individual chromosomes, resulting in each gamete having n=2 chromosomes, as stated in choice B. This process halves the chromosome number overall while ensuring haploid gametes. A tempting distractor is choice C, which claims n=4 per gamete, arising from the misconception that chromatids are only counted as chromosomes in meiosis I and not after separation in meiosis II. For meiosis questions, visualize the chromatid movements in each division to accurately count chromosomes in the final gametes.
In a diploid organism with $2n=6$, a cell is observed in anaphase I with three chromosome pairs moving to opposite poles; sister chromatids remain joined at centromeres. After this division completes and cytokinesis occurs, each resulting cell contains one chromosome from each homologous pair, but each chromosome still consists of two sister chromatids. Which outcome is most likely for the chromosome number and chromatid state in each daughter cell immediately after meiosis I?
Each daughter cell is haploid with $n=3$ chromosomes, each composed of a single chromatid.
Each daughter cell has $n=6$ chromosomes because homologs separate before sister chromatids.
Each daughter cell is haploid with $n=3$ chromosomes, each still composed of two sister chromatids.
Each daughter cell is diploid with $2n=6$ chromosomes, each composed of a single chromatid.
Each daughter cell is diploid with $2n=6$ chromosomes, each still composed of two sister chromatids.
Explanation
This question assesses the skill of analyzing meiosis by focusing on chromosome segregation and ploidy after meiosis I. In a diploid organism with 2n=6, anaphase I involves the separation of three homologous pairs to opposite poles, with sister chromatids remaining joined, resulting in each daughter cell receiving one chromosome from each pair. Since the chromosomes are still composed of two sister chromatids, the cells are haploid (n=3) but with duplicated chromosomes, matching choice A. This reflects the reduction division of meiosis I, where ploidy halves while chromatids stay intact until meiosis II. A tempting distractor is choice C, which incorrectly states each chromosome has a single chromatid, stemming from the misconception that sister chromatids separate in meiosis I rather than homologs. For meiosis questions, always track both the number of chromosomes and the chromatid composition separately to determine ploidy and cell state.
A diploid organism has $2n=8$. During meiosis I in one spermatocyte, homologous chromosomes separate normally except one homologous pair fails to separate at anaphase I, with both homologs moving to the same pole. Meiosis II proceeds normally, separating sister chromatids. Which set of gametes is most likely produced from this cell?
Four gametes each with $n=4$, because meiosis II corrects nondisjunction from meiosis I.
Two gametes with $n=5$ and two gametes with $n=3$, due to homolog nondisjunction in meiosis I.
Four gametes each with $n=2$, because chromosome number is reduced twice across meiosis I and II.
Two gametes with $n=4$ and two gametes with $n=6$, because sister chromatids fail to separate.
All gametes with $n=4$, but each has duplicated chromosomes because chromatids remain attached.
Explanation
This question assesses the skill of analyzing meiosis by examining the consequences of nondisjunction during meiosis I on gamete chromosome numbers. With 2n=8, nondisjunction of one homologous pair in anaphase I means one daughter cell receives both homologs (5 chromosomes total) and the other receives none for that pair (3 chromosomes total). In meiosis II, sister chromatids separate normally, so the cell with 5 chromosomes produces two gametes each with n=5, and the cell with 3 produces two with n=3. This results in two gametes with n=5 and two with n=3, reflecting the aneuploidy from the meiosis I error. A tempting distractor is choice A, which wrongly claims meiosis II corrects the nondisjunction, misconstruing that meiosis II only separates chromatids and cannot redistribute missing or extra homologs from meiosis I. A transferable strategy for meiosis questions is to count chromosomes at each stage, noting how errors in one division propagate without correction in the subsequent division.
In a cell entering meiosis with $2n=4$, each chromosome replicates to form sister chromatids. During anaphase I, homologous chromosomes separate to opposite poles while sister chromatids remain joined. The cell completes telophase I and cytokinesis, forming two cells. Which description best accounts for the chromosome composition in each daughter cell after meiosis I?
Each daughter cell is diploid with four replicated chromosomes, because sister chromatids did not separate.
Each daughter cell is haploid with two replicated chromosomes, because homologs separated but chromatids stayed joined.
Each daughter cell is haploid with two unreplicated chromosomes, because DNA replication is reversed in meiosis I.
One daughter cell is haploid and the other is diploid, because homologs assort independently.
Each daughter cell is diploid with four unreplicated chromosomes, because homologs separated.
Explanation
This question assesses the skill of analyzing meiosis by examining chromosome composition after meiosis I in a cell with 2n=4. After DNA replication, each of the four chromosomes consists of two sister chromatids, and in anaphase I, homologous chromosomes separate to opposite poles while sister chromatids remain attached. Thus, each daughter cell receives two chromosomes (one from each homologous pair), each still composed of two joined sister chromatids. These daughter cells are haploid (n=2) but contain replicated chromosomes ready for meiosis II. A tempting distractor is choice D, which incorrectly states each cell is diploid with four replicated chromosomes, misunderstanding that homolog separation in meiosis I reduces the chromosome number to haploid despite chromatids remaining joined. A transferable strategy for meiosis questions is to distinguish between chromosome number (based on centromeres) and replication state, tracking what separates in each division.
A diploid cell has $2n=4$ with two homologous pairs (pair 1 and pair 2). At metaphase I, the orientation of each homologous pair toward opposite poles is independent of the other pair. The cell completes meiosis I and II without errors and without considering crossing over. Which outcome is most likely regarding the chromosome combinations in the resulting gametes from many such cells?
Only one gamete type occurs, because meiosis produces genetically identical haploid cells like mitosis.
Four gamete types occur only if homologs fail to pair during prophase I.
Exactly eight gamete types occur, because each chromatid assort independently during meiosis I.
Exactly two gamete types occur, because meiosis always separates maternal from paternal chromosomes together.
Exactly four gamete types occur, because independent assortment can produce $2^2$ combinations of homologs.
Explanation
This question assesses the skill of analyzing meiosis by examining independent assortment in a cell with 2n=4 and two homologous pairs. Without crossing over, the orientation of each pair at metaphase I is independent, allowing for $2^2$ = 4 possible combinations of maternal and paternal chromosomes in gametes. Across many cells, all four combinations occur as meiosis I separates homologs and meiosis II separates chromatids, producing diverse but predictable gamete types. This reflects the principle that each pair assorts independently, generating genetic variety. A tempting distractor is choice C, which wrongly states eight gamete types due to independent chromatid assortment, misconstruing that chromatids do not assort independently in meiosis I but stay with their homolog. A transferable strategy for meiosis questions is to calculate gamete diversity using $2^n$ for n homologous pairs, excluding crossing over unless specified.
A diploid cell with $2n=4$ completes DNA replication before meiosis. At metaphase I, both homologous pairs align. In anaphase I, homologous chromosomes separate normally. After meiosis I, each daughter cell contains two chromosomes, each still composed of two sister chromatids. Without another round of DNA replication, meiosis II occurs. Which change in chromosome structure occurs during anaphase II?
Tetrads form as homologous chromosomes synapse and exchange segments.
Homologous chromosomes pair at the metaphase plate to ensure identical gametes.
Sister chromatids separate when centromeres divide, forming single-chromatid chromosomes.
Chromosomes replicate again, restoring two sister chromatids per chromosome.
Homologous chromosomes separate, reducing chromosome number from $2n$ to $n$.
Explanation
This question focuses on the specific events of anaphase II in meiosis. After meiosis I, each daughter cell has the haploid number of chromosomes (n=2), but each chromosome still consists of two sister chromatids joined at the centromere. During anaphase II, centromeres divide and sister chromatids separate, converting each double-chromatid chromosome into two single-chromatid chromosomes that move to opposite poles. This is the same mechanism as mitotic anaphase but occurs in haploid cells rather than diploid cells. Students choosing A confuse this with meiosis I events, incorrectly thinking homologs separate again in meiosis II. To distinguish meiosis I from meiosis II events, remember that homolog separation (meiosis I) reduces chromosome number while sister chromatid separation (meiosis II) changes chromosome structure from double to single chromatids.
A diploid cell ($2n=6$) undergoes meiosis. During prophase I, synapsis occurs normally, but crossing over is experimentally prevented. Homologous chromosomes still align as tetrads at metaphase I and segregate to opposite poles in anaphase I. Sister chromatids separate in meiosis II. Which outcome is most likely regarding the chromatid composition of the resulting gametes?
Gametes contain chromatids that match the original parental chromatids for each chromosome.
Gametes contain extra chromosomes because preventing crossing over causes nondisjunction.
Gametes contain diploid sets because crossing over is required for homolog separation.
Gametes contain recombinant chromatids because synapsis alone causes segment exchange.
Gametes contain identical chromatids because meiosis II is equivalent to mitosis.
Explanation
This question examines meiosis when crossing over is prevented but synapsis still occurs. Without crossing over, homologous chromosomes pair normally during prophase I but do not exchange DNA segments between non-sister chromatids. Each chromatid remains identical to its original parental form throughout meiosis. When homologs separate in meiosis I and sister chromatids separate in meiosis II, each gamete receives chromatids that exactly match the original parental chromosomes without any recombination. Students choosing A incorrectly believe synapsis alone causes DNA exchange, not recognizing that crossing over is a separate process requiring specific enzymatic machinery. The key insight is that crossing over and synapsis are distinct processes: chromosomes can pair without exchanging segments, producing gametes with purely parental chromosome combinations.
A meiotic cell is experimentally treated so that spindle fibers fail to attach to the kinetochores of one homologous chromosome pair during meiosis I. The other homologous pairs attach and segregate normally. Cytokinesis still occurs. Meiosis II proceeds with normal spindle attachment to any chromosomes present in each cell. Which outcome is most likely for the distribution of that affected homologous pair among the gametes?
Each gamete receives one homolog because spindle attachment is only required in meiosis II.
Two gametes receive both homologs and two gametes receive neither homolog.
All gametes lack that chromosome because unattached chromosomes are excluded from nuclei.
Gametes receive four copies because chromosomes replicate again between meiosis I and II.
All gametes receive both homologs because homologs remain paired through meiosis II.
Explanation
This question analyzes the consequences of spindle attachment failure for an entire homologous pair during meiosis I. Without spindle attachment, the affected homologous pair cannot separate and both homologs remain together, likely moving randomly to one pole or remaining in the middle during cytokinesis. If both homologs end up in one daughter cell, that cell will have both homologs while the other cell has neither. During meiosis II, the cell with both homologs produces two gametes each receiving both (as they separate as sister chromatids), while the cell lacking those chromosomes produces two gametes with neither. Students choosing B incorrectly assume unattached chromosomes are always lost rather than potentially being included in one daughter cell. When spindle attachment fails for entire homologous pairs, the result is typically all-or-nothing distribution creating reciprocal imbalances.